It is known to use loop seams and spiral seams in openable clothings, such as dryer fabrics, for paper and cellulose machines, the opposite seam edges of the clothing each having a row of loops. When the clothing is to be installed in the machine, this is carried out with the seam being open, whereupon the seam edges are moved together and the seam loops of one row are meshed with the seam loops of the other row. Then the seam is joined by a so-called pintle wire being passed through the thus intermeshed rows of loops. The principle of such seamed clothings is described in more detail in an article by T. Branham in Tappi Journal (USA), June 1994, Vol. 77, No. 6, pp 285-288.
In a so-called warp loop seam, the rows of loops are formed of extended edge loops of warp yarns in the fabric structure of the clothing. In a so-called spiral seam, each row of loops is instead formed of a separate, preformed yarn spiral, which is extended along and attached by means of machine direction yarns, such as warp yarns, to the seam edge of the clothing. The yarn spiral is normally attached to the seam edge by being, after completion of the weaving, spliced into the seam edge by means of a special splicing machine. Alternatively, the spiral can be attached to the clothing by a number of cross-machine direction yarns being ravelled a distance from the seam edge, whereupon the loops of the spiral are inserted into the thus formed looser edge portion. Then the edge is folded back over itself and is attached to -he clothing, for instance, by using a sewing-machine. Independently of how the spiral is attached, the clothing comprises two spirals, one along each seam edge, which, when joining together the fabric, are meshed with each other like a zipper so as to be joined together by means of a pintle wire or the like.
Using such separate spirals is, in some cases, preferred to woven warp loops, since the shape of seam loops formed of a spiral is less dependent on the weaving technique.
A seam is generally a critical part of a seamed machine clothing, since a uniform paper quality, low marking and an excellent runnability of the clothing require a seam which is as similar as possible to the rest of the clothing in respect of properties such as thickness, structure, strength, permeability etc. These requirements are especially important in dryer fabrics in drying positions with suction rolls.
However, there are problems in prior-art spiral seams if the yarn spirals are bound to the seam edges by means of so-called flat filament yarns. In many cases, a dryer fabric is today woven of flat filament yarns, at least in its machine direction. By flat filament yarns are below meant yarns of non-circular cross-section. The cross-section of a flat filament yarn can be essentially rectangular, elliptic, an elongated polygon or combinations thereof. In general, the cross-section of the flat filament yarn has a great cross-sectional dimension and a small cross-sectional dimension. In the ready dryer fabric, the flat filament yarn extends with its great cross-sectional dimension substantially in parallel with the principal plane of the fabric.
Flat filament yarns are preferred to round yarns of circular cross-section, on the one hand because a flat filament yarn does not build up in the thickness direction to the same extent as a round yarn, i.e. it results in a thinner clothing and, on the other hand, since a flat filament yarn yields a better contact with the web that is to be dewatered. A further advantage of flat filament yarns is that the wear potential of the fabric increases. Thus, the fabric maintains its surface structure, which results in a reliable and stable operation.
The flat filament yarn thus extends with its great cross-sectional dimension in parallel with the principal plane of the fabric, but in the area in which the flat filament yarn, for securing the yarn spiral to the seam edge, passes between two neighboring spiral loops (seam loops) and turns the spiral yarn over so as to bind thereto, the flat filament yarn will, however, normally occupy a different, less favorable orientation, in which the great cross-sectional dimension of the flat filament yarn instead makes an angle with the principal plane of the fabric. The situation at the seam edge thus is that the elongate cross-section of the flat filament yarn "places itself on edge" adjacent the yarn spiral. As a result, the flat filament yarn projects in the same area too high up above the surface of the fabric, which may lead to web marks and/or yarn wear at the risk of yarn breakage. This unfavorable situation is schematically illustrated in FIG. 1A of the enclosed drawing, which shows how a rectangular cross-section of a flat filament yarn, which binds around a known yarn spiral, adapts poorly to the round "valley" of the spiral between two neighboring seam loops. In the two valleys to the right in FIG. 1A, the flat filament yarn has the above-mentioned unfavorable, inclined orientation.
It is also apparent from FIG. 1A that there is a further problem in the prior-art technique, illustrated by the warp yarn to the left in the Figure. This warp yarn has unsatisfactory contact with the yarn spiral, and therefore the binding points between the warp yarn and the spiral are not optimal. Owing to this unsatisfactory contact, the above-mentioned unfavorable displacement and turning of the warp yarns can arise more easily.
It will be appreciated that the above-mentioned problems will also be the more pronounced the wider the flat filament yarn selected.
The object of the present invention is to obviate the above problems and drawbacks of the prior-art technique.
Thus, the object of the invention is to ensure, in a machine clothing having a spiral seam, that optimum binding points are obtained between the clothing and the yarn spiral in the seam area, and that unfavorable incorrect orientation of the machine direction yarns in the seam area is prevented.